In order to automatically track the position of a satellite, satellite receivers are installed in moving objects such as vehicles, ships or the like. A device comprises means for adjusting azimuth angle and elevation angle of the antenna such that the position of the satellite is automatically tracked without adjustment of the wave-receiving angle of the antenna.
The use of such a two-axis system on a moving vehicle requires, in order to maintaining contact with the satellite, that the direction vector all the time is kept parallel. To accomplish this with such a two-axis system roll movements are compensated by means of an azimuth motor which imparts a rotational motion to the antenna about an azimuth axis and an elevation motor which imparts a rotational motion to the antenna about an elevation axis. Such systems are well known.
However, at an elevation angle above 45° the response between roll motion and required compensation in azimuth and elevation direction becomes too large. At an elevation angle of 45° the response is 1:1, i.e. a roll motion of x°/s needs compensation in azimuth direction of x°/s. At an elevation angle larger than 45° the response increases and at an elevation angle of 90° the response is infinite. At an elevation angle of 90° there is thus a singularity. This above mentioned problem is referred to as the zenith problem.
In order to take the polarization in to consideration such systems further comprises means for adjusting the polarization angle of the transceiver head of the antenna, by means of imparting a rotational motion to the transceiver head about a polarization axis. This improves the possibilities of communicating with a satellite such that it is possible to both receive and transmit signals, also during movement, during conditions not involving the above mentioned zenith problem. However, at elevation angles above 45° involving roll motions such a three axis system does not work due to the above mentioned limitation in response. The requirements for transmitting/broadcasting are strict and during movement in these conditions such a system does not meet these requirements, as there will be noise transmitted to adjacent channels due to the limitation in tracking the antenna. Thus, the vehicle would have to stand still. However, in e.g. a war zone it may be desired to be able to transmit during movement when tracking a satellite at an elevation angle above 45°, in rough terrain involving roll motions. Also in other applications such as television broadcasting, fire fighting and the like the possibility of transmitting during movement in such conditions may be requested.
U.S. Pat. No. 7,095,376 discloses a device for controlling a satellite tracking antenna having a three-axis system, an azimuth axis, an elevation axis and in addition a tilt axis, said device being arranged on an aeroplane. The tilt axis is connected to the elevation axis in such a way that the rotational freedom of motion of the antenna about the tilt axis is dependent on the elevation angle such that it corresponds to an azimuthal rotational motion at an elevation angle of 0° and a roll rotation at an elevation angle of 90°, thus solving the Zenith problem. The change in roll, pitch and heading of the aircraft can be determined by an inertial navigation system.